The present invention relates to a semiconductor device such as a phototransistor configured to output a photocurrent upon light irradiation and also relates to an imaging apparatus using the semiconductor device.
A photodiode is often used as a photodetector since it can be manufactured by a simple process and can output photocurrent with constant sensitivity. However, since photocurrent obtained by light irradiation is weak, the photodiode has to have a large light-receiving area in order to have better light sensitivity under a low illuminance. A phototransistor having a bipolar transistor structure is characterized by its capability of, using the property of its bipolar transistor structure, amplifying photocurrent obtained by a collector-base photodiode before outputting it from an emitter or a collector of the phototransistor. To utilize this characteristic, a phototransistor is used for an imaging apparatus to achieve improvement in sensitivity.
When the level of photocurrent amplification is increased using the phototransistor, a large output signal can be obtained with a low illuminance, facilitating signal processing. The higher the illuminance, the larger the output current becomes. However, in a case where phototransistors are arranged in an array in an imaging apparatus, when the amount of current is large, resistance at a pixel address selector becomes problematic to make it difficult to accurately read a signal. As a result, it has been impossible to increase the gain and to increase the dynamic range of detectable light intensity at the same time. Moreover, special imaging and the like require partial correction of the brightness of an image. The partial correction of the brightness of an image is implemented by software processing. However, in software processing, when signal output cannot be obtained in real time, a delay occurs for the period of time for information processing. Furthermore, the amount of information obtained is limited because the dynamic range of the phototransistor is limited. Accordingly, what is needed is an imaging element capable of varying the current gain of a phototransistor and thereby varying the level of obtained photocurrent according to the light intensity.
Japanese Patent Application Publication No. 2010-225797 (referred to as Patent Document 1 below) discloses an example of a transistor structure capable of varying the current gain. This is achieved by making a lateral double-diffused MOS structure operate in a bipolar transistor manner (see FIG. 1). In the example illustrated in FIG. 1, a collector 3 made of an N-type diffusion layer, a base 5 made of a P-type diffusion layer, and an emitter 7 made of an N-type diffusion layer are formed in a P-type semiconductor layer 1, to form a bipolar transistor. The bipolar transistor is provided with a gate electrode 11 opposing the base 5 and the collector 3 with a gate insulation film 9 interposed between the gate electrode 11 and surfaces of the base 5 and the collector 3. The collector 3 is connected with collector wiring 13. The base 5 is connected with base wiring 15. The emitter 7 is connected with emitter wiring 17. The gate electrode 11 is connected with gate electrode wiring 19. The wiring 13, 15, 17, and 19 are electrically insulated with each other. Additionally, this document also shows how the current gain varies through application of voltage to a gate electrode (see FIG. 2). Refer also to Japanese Patent No. 3071161.
The inventor of the present invention focused on the fact that a transistor having a structure capable of varying its current gain can operate as a phototransistor by using photocurrent obtained by light irradiation as a base current. In other words, by varying the level of voltage applied to the gate electrode, the current gain can be varied likewise for photocurrent amplification.
However, to improve resolution which determines image quality, the larger the light-receiving cell, the better. A phototransistor having a bipolar transistor structure has diffusion regions of emitter, base, and collector from the semiconductor surface in its depth direction. When used in an imaging apparatus, each phototransistor can be operated with common collector, but has to have isolated base. Since base diffusion spreads laterally, cells have to be located away from each other. In array arrangement, reducing the area of the array is difficult because enough space has to be secured between base diffusion regions.
In addition, since the regions of the bipolar transistor which are used to make the current gain is variable is only portions opposed to the gate electrode and is therefore of small area, only a limited effect can be obtained. Further, being located on this bipolar transistor region, the gate electrode hinders incidence of light.